Oh no!! The poor little guy. A similar incident happened to me and my roosters a couple months ago. It does look alot worse then it actually is, at least in my case it was. However, in my case both my roosters were pretty badly beat up but their eyes were not swollen.
Well, other than bringing him to a veterinarian (which would be considered the gold standard so to speak) you are doing everything you really can do. Besides from his injuries, the poor little guy is probably feeling mentally very sad, seeing as he was just severely beat up by his own sons (I hate how awful roosters are to each other

) Try to encourage him to drink and eat, and watch how much he drinks and eat. If he has a favorite treat, try giving him the favorite treat. Monitor him every hour or so to make sure there are no changes in his condition and to track whether he is eating or drinking. If he isn't you are going to have to use other methods of getting fluids into him. If you have electrolytes or probiotics, these would be beneficial for him as well right now.
As for the long term, if he recovers, whatever you do, do not put him back out together with the other roosters. They will kill him next time. You are going to have to permanently keep them separated. What I ended up doing with mine is I rotate them. So for one day one rooster goes out with the hens and the other stays locked up. Then the next day I switch them. However, seeing as you have more roosters, this may get tricky. You are going to have to probably build another enclosure :/
Here are links to photos on my Instagram of how my roosters looked after their fights -
https://www.instagram.com/p/BNaOnKlhpun/?taken-by=quackersandfernie&hl=en
https://www.instagram.com/p/BKPH1xVhimI/?taken-by=quackersandfernie&hl=en
https://www.instagram.com/p/BKMg9CMBUuq/?taken-by=quackersandfernie&hl=en
and here is an example of my temporary 'jail' I made out of a dog kennel (but made sure to cover all the possible ways the other rooster could possibly access inside, as they will fight through the cage) -
https://www.instagram.com/p/BO8OMnIgkv8/?taken-by=quackersandfernie&hl=en
Good luck, I really hope your little guy makes it :/